DEFRA
RESEARCH CONTRACT
“RESERVOIR SAFETY – FLOODS AND RESERVOIR SAFETY INTEGRATION”
FINAL REPORT
Volume 1 of 3
MAIN REPORT
Ref. XU0168 Rev A05
August 2002
Building Research EstablishmentGarston
Watford WD2 7JR / Hill Park Court
Springfield Drive
Leatherhead
Surrey, KT22 7NL
ENGLAND
FLOODS AND RESERVOIR SAFETY INTEGRATION
VOLUME I – MAIN REPORT DEFRA RESEARCH CONTRACT
DOCUMENT HISTORY RECORD
Rev / Date / Details / By / Checked / ApprovedA01 / 14th Nov 01 / Issue to client for Milestone 3 as Interim Report / AJB/PT/MF/PG / JDG / AJB
A02 / 30th Jan 2002 / Issue internally for Internal Review Group / AJB/PT/MF / JDG / AJB
A03 / 20th Feb 2002 / Issue to client for Milestone 4a/5a as Preliminary Draft Final Report / AJB/ MF / JDG / AJB
A04 / 8th May 2002 / Issue to client for Milestone 4b/5b as Draft Final
(changes to Sections 1 to 6 & 9 of Revision A03 shown by ‘track changes’ line in margin) / AJB / JDG / AJB
A05 / July 2002 / Issue as FINAL / AJB / JDG / AJB
The authors of this report are employed by Brown & Root. The work reported herein was carried out under a Contract placed on 12th April 2001 by the Secretary of State for the Environment, Transport and the Regions. Any views expressed are not necessarily those of the Secretary of State for the Environment, Transport and the Regions.”
©Copyright 2002.
REPORT FEEDBACK FORM
Client: / DEFRA
Project: / Research Contract : “Reservoir safety – Floods and reservoir safety integration”
Project Ref./ Report Document No : / XU0168 Final Report
Report Title : / As Document History Record
Report Indicators / 5
Excellent / 4
Good / 3
Satisfactory / 2
Poor / 1
Unsatisfactory
Understanding of Client Requirements
English, Style, Presentation & Collation
Quality of Report Content & Information
Clear Conclusions & Recommendations
Delivery on Time
Overall Client Satisfaction with Report
(please tick as appropriate)
Client Comments:Name: Title: Date:
Please return to:
Colin Ray - Director of Engineering
Brown & Root Services, Hill Park Court
Springfield Drive, Leatherhead, Surrey KT22 7NL
Tel 01372 862917 Fax 01372 863355` Form BRS 401/7
VOLUME PLAN
Volume1 / Main Report
2 / Appendices A to H : Supporting Documentation
A: References ordered by Author
B: References ordered by Subject/ Relevance
C: National dam database (NDD)
D: Characteristics of dams world-wide
E: Methods currently used for risk assessment of dam safety worldwide
F : Estimation of floods and other external threats
G: Techniques for estimating probability of failure of dams
H: Techniques for assessing consequences of dam failure and tolerable risk
3 / Appendices J & K : The Integrated System
J: The Integrated System for embankment dams
K: Trial of the Integrated System on 10 dams
VOLUME 1 : CONTENTS
EXECUTIVE SUMMARY ii
USER GUIDE TO THE REPORT i
1 INTRODUCTION 1
1.1 Background 1
1.2 Objectives 1
1.3 Terminology 2
1.4 Background Issues 2
1.4.1 General 2
1.4.2 UK dams 2
1.4.3 Strategic drivers for an Integrated System in the UK 4
1.4.4 National Dams Database (BRE database) 5
2 DEFINITIONS AND historical PERFORMANCE OF UK DAMS 7
2.1 General 7
2.2 Definitions 7
2.2.1 Issues 7
2.2.2 Event Trains 8
2.2.3 Failure 10
2.2.4 External and Internal threats 11
2.2.5 Terminology and independent threats adopted for this project 12
2.2.6 Probability of Failure 15
2.2.7 Risk analysis, evaluation, assessment, control and management 17
2.3 Methodology for assessing Performance of UK dams using NDD 18
2.3.1 Structure of NDD 18
2.3.2 Approach to interrogation of NDD 18
2.3.3 Results of searches 20
2.4 Historical performance of UK embankment dams 22
2.4.1 Introduction 22
2.4.2 Failure in operation 22
2.4.3 Incidents in operation 23
2.4.4 Failures and incidents during first five years (wear-in) 27
2.4.5 Frequency of occurrence of external threats, and definition of failure 28
2.4.6 Confidence limits and value of database 30
2.4.7 Possible current annual probability of failure 30
2.5 Historical Performance of UK Concrete dams in operation 33
2.6 Historical performance of UK Service Reservoirs in operation 33
2.7 Conclusions 34
3 CONSEQUENCES OF FAILURE 36
3.1 General 36
3.2 Combination of breach conditions and distribution of population 36
3.3 Incremental damage due to floodwave 38
3.4 Likely Loss of Life 38
3.5 Physical Damage and Economic Loss 39
3.6 Emergency Plans 41
3.6.1 Legal requirement for emergency plans 41
3.6.2 Definition and content of emergency plan 41
3.6.3 Effectiveness of emergency plans 42
3.7 Summary 42
4 TOLERABLE RISK 44
4.1 Introduction 44
4.2 Definitions of tolerable risk 44
4.2.1 Historical development 44
4.2.2 FN curves 45
4.2.3 Other measures of tolerability of failure 45
4.2.4 The ALARP principle 46
4.2.5 Reducing Risk, protecting people (R2P2, 2001) 46
4.2.6 HSE Regulating higher hazards: exploring the issues 47
4.2.7 Summary 48
4.3 Tolerable risk for UK dams 48
4.3.1 General 48
4.3.2 Current UK design standards vs. Hazard Class 48
4.3.3 Application of FN curves to UK dams 51
4.3.4 Discussion 51
4.3.5 Summary of Conclusions 54
5 FEASIBILITY OF ESTIMATING OVERALL PROBABILITY OF FAILURE 56
5.1 Introduction 56
5.2 Definition of an Integrated System 57
5.2.1 Basis of an Integrated System 57
5.2.2 Level of detail of assessment 58
5.2.3 Required Output 59
5.2.4 Input data to and parameters for an Integrated System 61
5.2.5 Precedent 62
5.3 Overview of Techniques for assessing the safety of a dam 65
5.3.1 Introduction 65
5.3.2 Available techniques 65
5.3.3 Definitions of Expert and Engineering judgement 66
5.3.4 Annual probability of failure by Analysis 67
5.3.5 Annual probability of failure from Historical Performance 68
5.3.6 Annual probability of Failure using Expert Judgement 72
5.3.7 The role of Engineering Judgement 73
5.3.8 Order of preference of techniques 74
5.4 Feasibility of an Integrated System 74
5.4.1 Introduction 74
5.4.2 External threats 74
5.4.3 Internal threats 75
5.4.4 Conclusions 76
6 PROPOSE AN INTEGRATED SYSTEM 78
6.1 Introduction 78
6.2 General Issues relating to estimating probability of failure 79
6.2.1 Introduction 79
6.2.2 Criticality 79
6.2.3 Joint probability 80
6.2.4 Use of Probable Maximum Precipitation 82
6.2.5 Estimation of extreme events 86
6.3 External threats 86
6.3.1 General 86
6.3.2 Extreme rainfall (Sheets J.6, J.13) 87
6.3.3 Wind (Sheets J.7, J.14) 89
6.3.4 Failure of reservoir in cascade upstream (Sheets J.8, J.15) 90
6.3.5 Seismic events (Sheets J9, J.16) 91
6.3.6 Other External Threats and Mechanisms of deterioration (Sheet J.12) 93
6.4 Internal threats (Sheets J.10, J.11 and J.17 to J.19) 94
6.4.1 General 94
6.4.2 Defining the anchor point(s) for the worst condition dam 94
6.4.3 Defining the anchor point(s) for the best condition dam 96
6.4.4 Descriptors for Intrinsic and Current condition 99
6.5 Consequences of failure (Sheets J.20 to J.22) 101
6.5.1 Breach hydrograph and downstream attenuation 101
6.5.2 Combination of breach conditions and distribution of population 101
6.5.3 Estimating population at risk and likely loss of life 101
6.5.4 Physical Damage and economic loss due to dam breach flood 102
6.5.5 Failure of dams in cascade 102
6.6 Assessing tolerable annual probability of failure (Sheet J.23) 103
6.6.1 General 103
6.6.2 Hazard Class 104
6.6.3 FN curves 105
6.6.4 ALARP 106
6.6.5 Limits adopted for Integrated System 106
6.7 Summary 106
7 RESULTS OF THE TRIAL OF THE PROTOTYPE Integrated System 108
7.1 Introduction 108
7.2 Criteria to judge the success of the Integrated System 108
7.3 Description of trial 109
7.4 External threats 111
7.4.1 General 111
7.4.2 Extreme rainfall 111
7.4.3 Wind 113
7.4.4 Failure of reservoir in cascade upstream 114
7.4.5 Seismic 115
7.4.6 Summary 115
7.5 Internal threats 116
7.5.1 General 116
7.5.2 Internal Stability (Embankment) 116
7.5.3 Internal Stability (appurtenant works) 117
7.5.4 Discussion 118
7.6 Overall annual probability of failure 120
7.6.1 General 120
7.6.2 Comparison with estimated annual probability of failure of UK dams 120
7.6.3 Other mechanisms of deterioration for four core threats, and other external threats 121
7.7 Assessment of the consequences of dam break 122
7.7.1 General 122
7.7.2 Breach discharge and downstream attenuation 122
7.7.3 Effect of reservoir in cascade 123
7.7.4 PAR and LLOL 123
7.7.5 Physical damage and economic loss 123
7.7.6 Summary 123
7.8 Sensitivity Study of risk posed by dam 125
7.9 Tolerability of risk (Risk assessment) 127
7.9.1 General 127
7.9.2 Hazard Class 127
7.9.3 FN curves 127
7.9.4 ALARP 128
7.9.5 Summary 129
7.10 Discussion 131
7.10.1 Overview of output 131
7.10.2 Success and value of the Integrated System 131
7.10.3 Debatable Issues 134
7.10.4 Future for the Integrated System 135
7.11 Summary of Conclusions from trial 138
8 NEED FOR REVISION OF PUBLISHED ENGINEERING GUIDES 140
8.1 Introduction 140
8.2 Strategy for defining ‘Design Standards’ 140
8.3 Standing and need for revision of existing Guides 142
8.3.1 General 142
8.3.2 Floods and Reservoir safety (FRS) 143
8.3.3 Seismic risk to dams in the UK 144
8.3.4 Safety of embankment dams 144
8.3.5 Investigating embankment dams 145
8.3.6 Valves, pipework and associated equipment in dams 145
8.3.7 Risk management for UK Reservoirs (RMUKR) 145
9 GLOSSARY 146
9.1 Acronyms 146
9.2 Symbols 148
9.3 Terminology 149
List of Tables
Table 1.1 Terms with related, but different meanings used in this research contract 2
Table 1.2 Chronology of main events relating to dam safety in UK 3
Table 1.3 Distribution of physical characteristics of all UK dams 3
Table 1.4 Distribution of dam type with height and age 3
Table 1.5 Principles of “permissioning” regimes (as HSE, 2000) 5
Table 2.1 Precedent for terminology for stages in train of events necessary to cause dam failure, and number of items at each stage 9
Table 2.2 Subdivision of internal threats 12
Table 2.3 Terminology used in this research project 13
Table 2.4 Matrix showing relationship between threats and failure modes 14
Table 2.5 Factors affecting accuracy and application of estimated probabilities 16
Table 2.6 Approach to search of NDD 19
Table 2.7 Results of interrogation of NDD by dam type and failure mode 21
Table 2.8 Estimated annual probabilities and relative proportion of failure modes from Table 2.7 21
Table 2.9 Summary of analysis of incidents 24
Table 2.10 Proportion of failure modes for UK dams 29
Table 2.11 Proportions of failure modes in the published literature 29
Table 2.12 Confidence limits on estimated annual probabilities for embankment dams in operation 1975 to 2000 32
Table 2.13 Possible current annual probability of failure to UK dams, from assessment of threats 32
Table 3.1 Possible combinations of dam break and downstream population scenarios 37
Table 3.2 RMUKR impact scoring system 40
Table 4.1 Tolerability criteria based on annualised life loss 46
Table 4.2 Techniques that may be used to quantify tolerability of risk 48
Table 4.3 Current systems for evaluating acceptable risk from UK dams 50
Table 5.1 Output required from an Integrated System for an individual dam 59
Table 5.2 Summary of risk analysis levels (as Table 1 of McCann, 1998) 60
Table 5.3 Summary of existing attempts at an "Integrated System" and systems for “Portfolio risk assessment” 64
Table 5.4 Toolbox of possible ways of assessing the safety of dams 66
Table 5.5 Attributes of Engineering Judgement and Expert Judgement (Table I of Skipp & Woo, 1993) 67
Table 5.6 Possible correction factors in adjusting historical AP of failure by a mode of failure for all dams, to AP of failure due to a threat at a single dam 69
Table 5.7 Data relevant to correction of annual probability of failure to specific dam 70
Table 5.8 Steps in process of relating probability of failure to condition score for a threat 71
Table 5.9 Examples of probability mapping schemes (probability assigned to qualitative terms) 73
Table 6.1 Issues in the selection of the methodology of flood estimates for high hazard dams 85
Table 6.2 Other mechanisms of deterioration and threats that may be significant on a dam specific basis 93
Table 6.3 Data available which could inform the selection of anchor point AP of failure of good condition dams due to internal threats 97
Table 6.4 Assessed AP of anchor points defining AP of failure due to internal threats 98
Table 6.5 Matrix to determine Hazard Class (consequence category) of UK dams 104
Table 6.6 Acceptable AP of failure from FN curves (lower limit to tolerable region) 105
Table 7.1 Details of dams on which the trial of the Integrated System was carried out. 110
Table 7.2 Summary of the annual probability of failure from the trial of the Integrated System on ten dams 110
Table 7.3 Summary of Intrinsic and current condition scores for internal threats 110
Table 7.4 Consequences of failure and Hazard Class 124
Table 7.5 Sensitivity test on Dam 3 of methodology used in Integrated System. 126
Table 7.6 ALARP calculation for cost-to-save-a-statistical-life (CSSL) for proposed works to Dam 4 130
Table 7.7 Sensitivity study on Dam 3 of ALARP calculation for cost-to-save-a-statistical-life (CSSL) 130
Table 7.8 Assessment of value of prototype Integrated System to individual dams 132
Table 7.9 Assessment of overall value of the prototype Integrated System 132
Table 7.10 Overview of the structure and future for the prototype Integrated System 137
Table 8.2 Load cases normally considered in relation to seismic safety of dams (ICOLD Bulletin 72 – Selecting seismic parameters for large dams, 1989) 140
Table 8.1 Existing Engineering Guides 141
Table 8.3 Proposed new general approach in UK dam safety practice to definition of loading cases from external threats 142
Table 8.4 Indicative future format of revised Table 1 in FRS 144
List of Figures
Figure E.1 Figure giving overview of prototype Integrated System i
Figure 1.1 Process Diagram for Factors Relevant to the Integration of Possible Threats to Dam Safety 6
Figure 2.1 Overview of human influence in event train affecting dam safety 35